1. Field of the Invention
The present invention relates generally to methods of forming an ohmic electrode formed on an n type nitride-based compound semiconductor layer, and particularly to methods of forming an ohmic electrode having low resistance, hardly peeling off, and providing good ohmic contact.
2. Description of the Background Art
It has conventionally been known that a nitride-based compound semiconductor represented for example by InxGayAl1-x-yN can be used as a blue light emitting device, wherein 0≦x≦1, 0≦y≦1 and x+y≦1, and in recent years blue light emitting diodes, violet semiconductor lasers and the like are being studied. These light emitting diodes and semiconductor lasers need to externally receive a current. Accordingly, the material(s) of an ohmic electrode provided thereto and the technique used to form the same are significantly important.
A typical ohmic electrode formed on an n type nitride-based compound semiconductor layer is disclosed for example in Japanese Patent Laying-Open No. 7-45867. It is an ohmic electrode formed of a titanium (Ta) layer and an aluminum (Al) layer stacked and then annealed (hereinafter referred to as a “Ti/Al electrode”). Furthermore in the proceedings of the 60th annual meeting of the Japan Society of Applied Physics, page 302 (Lecture No. 3P-W-14) there is a report of an ohmic electrode superior in thermal stability to the Ti/Al electrode and allowed of being annealed in a wider temperature range of 400 to 600 degrees centigrade. According to the report, the ohmic electrode is formed of a hafnium (Hf) layer formed on an n type nitride-based compound semiconductor layer and an Al layer stacked thereon (hereinafter referred to as a “Hf/Al electrode”).
The above reported Hf/Al electrode, however, is formed by stacking Hf and Al layers without particularly controlling the concentrations of Hf and Al contained in the electrode. The Hf/Al electrode thus fabricated has a significantly rough surface and provides poor contact with the n type nitride-based compound semiconductor layer so that it may have high resistance. Furthermore, the electrode can peel off the semiconductor layer and thus fail to provide good ohmic contact.